The objective is to determine the efficacy of single drug therapy, ZDV or ddI, versus combination therapy (ddI/ZDV or ddC/ZDV) in preventing disease progression in HIV-infected persons with CD4 counts between 200 and 500. Secondary objectives include determining if there are differences in antiviral effect of these treatment regimen by quntitative measures of virus load, and determining whether viral resistance to these drugs can be prevented or delayed by using combination treatment rather than single agent therapy.